Recovery of oil sands bitumen

ABSTRACT

A method for improving the SWAG process for extracting the hydrocarbon, bitumen, from tar sands by injecting carbon dioxide into the reservoir. Said carbon dioxide will dissolve in the bitumen producing a solution having increased producibility greatly above that of the native bitumen. Recovery of the bitumen CO2 solution can be further enhanced by heat. Heat can be supplied by various means including elevating the temperature of the carbon dioxide prior to injection, injecting a mixture of carbon dioxide and steam or generating heat in-situ by including oxygen, preferably purified, with any of the above-mentioned injectants. This invention is ideally conducted through, but not limited to, the use of horizontal wells completed near the top and base of the reservoir.

REFERENCES

Patton, John T. et al, “Carbon Dioxide Well Stimulation: Part 1-A Parametric Study” J.Pet. Tech, 8-82

U.S. Pat. No. 4,390,068 Jun. 28, 1983 Patton, J. T., and Canfield, C. M. “Carbon Dioxide Stimulated Oil Recovery Process”

U.S. Pat. No. 3,516,495 i.6-23-70 Patton, J. T. “Recovery of Shale Oil”

Patton, J. T., “Enhanced Oil Recovery by Carbon Dioxide Flooding” Energy Communications, 1-78

Patton, J. T. and Saner, W. B., “CO2 Recovery of Heavy Oil: The Wilmington Field Test”, SPE 58^(th) Annual Technical Conference, 10-5-83

BACKGROUND OF THE INVENTION

This invention describes a novel and improved economical method of producing bitumen from a tar sand reservoir. Such a reservoir characterizes the Athabasca tar sands deposit in Alberta, Canada. Efforts were initiated early in the 1950's and continue today to develop a viable process for producing this resource.

Bitumen is a tar-like hydrocarbon whose high viscosity renders it relatively immobile. Initial commercial efforts for producing this bitumen involved:

-   -   1. Mining the tar-sand mixture and transporting it to a         processing plant.     -   2. Separating the bitumen from sand by slurring this mixture in         very hot water.     -   3. Recovering the oil-water thus separated from the sand.     -   4. Dewatering the oil by gravity separation, usually assisted by         deemulsifying agents in specially designed water-oil separators.

Disadvantages:

-   -   1. Process required tons of reservoir material to be mined and         transported to processing facility. This is very expensive and         wearing on the equipment.     -   2. Stable oil-water emulsions form that are expensive and         difficult to break.     -   3. Recovered oil phase is heavier than water and necessitated         specially designed separation equipment.     -   4. Enormous quantities of spent sand and polluted water require         disposal in a difficult to achieve environmentally acceptable         manner. Disposal methods are limited by numerous government         regulations.

It was immediately obvious from the start that an improved process was not only desirable but mandatory if this resource was to become economically viable. Research supported by the Province of Alberta suggested a steam enhanced in situ recovery alternate. In this research, steam would be injected into the oil sand to provide heat to reduce the bitumen viscosity and propel the hot fluids to an adjacent production well.

An alternative approach was developed by Exxon's Carter Research Laboratory in Tulsa, Okla. The result of this research culminated in the steam “Huff-n-puff” process invented by Exxon's, Dr. T. C. Boberg. Although showing much promise neither concept proved to be economically viable because injecting steam into the oil sand deposit through a conventional, vertical well was fraught with difficulty.

Shortly thereafter, Imperial Oil, Canada's Exxon affiliate, participated in limited field tests to evaluation in situ solvent extraction of bitumen. The process failed to become commercial because the low viscosity, high mobility, of the injected solvent caused most of the bitumen to be bypassed, not recovered. In addition, large quantities of expensive solvent remained in the reservoir.

To enhance hydrocarbon recovery in tight oil or gas reservoirs, industry, starting in about 1960, developed a new drilling technique. This technique, consisting of drilling producing wells horizontally through the reservoir, was found to be a significant breakthrough. For example, horizontal drilling is responsible for the commercialization of gas production from shale deposits such as the Devonian in central Texas.

The application of horizontal drilling to the tar sand reservoirs eventually became obvious. Drilling was easy and the shallow depth of the bitumen deposits minimized the costs. Although horizontal drilling made access to the tar sand reservoir practical, recovery of the bitumen remained uneconomic due to the high bitumen viscosity.

Additional techniques were investigated to lower bitumen viscosity and thus enhance recovery. Steam injection proved to be the most viable. In this process steam injection is attempted in horizontal wells transversing the top of the reservoir popularly referred to as steam assisted gravity drainage or SAGD for short. In this process bitumen, heated by the steam, drains to wells located in the reservoir below the level of injection where it can be subsequently recovered. Many oil companies including Devon Energy, ExxonMobil, Chevron and Conoco Phillips are aggressively commercializing this recovery process. Their efforts would be even more effective and efficient by incorporating the concepts described by this invention.

Each commercial application has encountered a major problem due to the low injectability of steam. This occurs because the steam condenses upon contact with the reservoir making further injection of steam more difficult. In addition, the low viscosity of the condensate allows it to bypass much of the bitumen. This severely limits the efficient transfer of heat to much of the reservoir, and hence, the production of bitumen is not optimally enhanced.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, the object of this invention to provide an improved and more profitable process for producing a hydrocarbon, like bitumen, from a tar sand deposit (reservoir). This invention is accomplished by injecting carbon dioxide, preferably heated, into said reservoir for the purpose of dissolution with and/or heating of the bitumen. Data presented in Table 1 demonstrate carbon dioxide dissolving in the bitumen produces a solution having a much lower viscosity than the bitumen. This phenomenon enables the resulting bitumen solution to flow more readily to one or more producing wells where it can be recovered from the reservoir.

TABLE 1 % Reduction in CO2 Dissolved in Bitumen, Oil Viscosity std ft3/bbl oil 60 54 73 140 95 280

These data were provided by the US Bureau of Mines Research Facility, Bartlesville, Okla.

The preferred embodiment of this invention is to facilitate the introduction of heat, i.e. steam injection, to the reservoir to further enhance the production of the bitumen. This is accomplished three ways: 1.) Injection of carbon dioxide, preferably heated, prior to steam injection. 2.) Injection of carbon dioxide in ad mixture with steam. 3.) Injection of purified oxygen, preferably greater than 50% oxygen, incorporated in either ways one or two above. The oxygen so injected will spontaneously oxidize a portion of the bitumen resulting in the production of carbon dioxide and an additional amount of steam. Steam produced by combustion will condense, and the water so formed will be beneficial by reducing the mobility of the carbon dioxide. This reduced mobility will increase the conformance of the injected gases. Additional heat can also be supplied by following the injection of carbon dioxide with steam injection. In this manner the beneficial effect of heat is both complimentary to and additive to the enhancement of bitumen production by dissolution with carbon dioxide.

There is a second potential benefit attributed to the presence of carbon dioxide dissolved in the produce bitumen. As currently produced, the native bitumen is surprisingly more dense than water which causes it to sink rather than float when separation from water is accomplished. It is hypothesized that carbon dioxide when dissolved in the bitumen will not only lower its viscosity but also the specific gravity. These combined phenomena will facilitate the separation of the phases and could minimize emulsion problems. This hypothesis is currently under investigation in the laboratory.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, it is the object of this invention to provide a novel and economical method of producing bitumen reservoired in a tar sand deposit. Production is enhanced by the injection of carbon dioxide. Such a process is feasible utilizing only a reasonable extension of techniques currently under development.

The solubility of carbon dioxide in bitumen is greater than most, if not all, non-hydrocarbon gases. In his SPE paper, “Carbon Dioxide Well Stimulation: Part I-A Parametric Study,” Patton, et al, reports a solubility of 140 SCF/bbl in very viscous crude such as bitumen. This fact coupled with the readily available low cost carbon dioxide as described in this invention provides potentially the first advantage of utilizing this invention for the economical production of bitumen.

A second unique advantage to the use of carbon dioxide is that it has the highest heat capacity of all common gases, 0.026 Btu/Cu.ft./degreesF. This heat capacity is 30% greater than water vapor and about 50% greater than nitrogen or air. The significance of this high heat capacity is that each cubic foot of injected carbon dioxide can transfer from 42% up to 100% more heat to the bitumen reservoir than other gases. It is therefore obvious that carbon dioxide is the preferred non-condensing gaseous injectant to supply heat to a bitumen reservoir.

A third, and possibly the most significant advantage of this invention, is the large reduction in air pollution, carbon dioxide emissions, to the atmosphere. By injecting carbon dioxide, normally exhausted to the atmosphere during the steam production process into the reservoir, emissions are greatly eliminated. By comparison with the air pollution generated by the mining process, an estimated 75% reduction in air pollution is realized.

In addition, any carbon dioxide produced with the bitumen is easily collected for re-injection. The total effect achieved by this invention will also include elimination of ground water pollution currently generated by the enormous settling basins used to dispose of the water in the mining process. The reduction in both air and water pollution are complementary and recommend the application of this invention.

The preferred embodiment of this invention is to initially inject hot carbon dioxide, above about 100 degrees F., into both production and injection wells. Steam may also be added to the carbon dioxide if desired. Injection of carbon dioxide into producing wells can vary from 5 to 500 SCF/ft of sand.

The introduction of carbon dioxide into injection wells, located toward the top of the reservoir, allows better distribution, and thus, promotes the dissolution of bitumen by carbon dioxide. In the injection cycle the new injection well could be back produced to accelerate the formation of the steam chest required during subsequent steam injection. At elevated temperatures the carbon dioxide provides heat to reduce bitumen viscosity. Carbon dioxide injection also supplies energy to drive mobilized bitumen into production wells located toward the base of the reservoir.

Carbon dioxide has a grossly lower specific gravity than bitumen. This fact will force the injected carbon dioxide to spread across the top of the formation due to natural gravity segregation. This greatly enhances the conformance and the subsequent bitumen dissolution by carbon dioxide. Utilizing these natural gravitational forces increases the efficacy of the process.

In an application where it is desirable to provide heat to the carbon dioxide by ad-mixture with steam, the similar laws of physics will apply. Introducing steam prior to, concurrently with, or following the carbon dioxide injection at the top of the reservoir will similarly achieve high conformance due to gravity segregation.

The exact composition of the injected gases into either the production or injection wells can vary widely. In all cases the stimulation provided by the carbon dioxide and steam has been calculated to be additive. However, in most cases this invention requires that the injected gas be at least 0.5 mol percent carbon dioxide. This percentage is calculated to be cumulative and does not refer to the injection at any specific instant in the performance of this invention. Initially, the injected gas can be about 100% carbon dioxide. Later in the life of the project the injected gases can be 100 mol percent steam. The preferred quality of the steam will be determined by other process variables including, but not limited to, the selected or available source of the steam.

The injection of purified oxygen to produce carbon dioxide in situ can be accomplished in several manners including, but not limited to, the following:

-   -   1. Injection of oxygen with carbon dioxide     -   2. Injecting oxygen concurrently with a mixture of steam and         carbon dioxide     -   3. Injecting oxygen concurrently with steam     -   4. Alternating oxygen injection with steam and/or carbon         dioxide.

Technique 4 has an additional advantage of minimizing any corrosion problems that might be caused by oxygen in the presence of liquid water, i.e. steam condensate and/or possibly carbon dioxide. 

What is claimed is:
 1. A process for increasing the production of bitumen from an oil sand reservoir by the injection of carbon dioxide into one or more wells penetrating said oil sand.
 2. A process described in claim 1 wherein the injection of carbon dioxide is followed by the injection of steam containing little or no carbon dioxide.
 3. A process described in claim 1 wherein purified oxygen is injected to produce carbon dioxide in situ.
 4. A process described in claim 1 wherein the injectant is a mixture of carbon dioxide and steam.
 5. A process described in claim 1 wherein the injectant is a mixture of carbon dioxide, steam, and oxygen.
 6. A process described in claim 1 wherein purified oxygen is injected to produce carbon dioxide in situ following the injection of carbon dioxide or steam.
 7. A process described in claim 1 wherein the carbon dioxide is heated to above 100 degrees Fahrenheit prior to injection.
 8. A process for enhancing the production of bitumen from a reservoir wherein carbon dioxide is injected near the top of the reservoir to decrease the bitumen viscosity and provide a portion of the energy required to displace the bitumen to one or more production wells located in the reservoir below the injection wells.
 9. A process for stimulating a new injection well consisting of injecting carbon dioxide into said injection well and then back producing the well to enhance the formation of a steam chest on subsequent steam injection cycles.
 10. A process described in claim 9 wherein the carbon dioxide is injected in ad mixture with steam. 